#include <iostream>
using namespace std;

class A {
	public:
		virtual void f() = 0;
		void g() {
			this->f();
		}
		A() {
		}
};

class B: public A {
	public:
		void f() {
			cout << "B:f()" << endl;
		}
};

int main() {
	B b;
	b.g();
	return 0;
}